1. Field of the Invention
The present invention relates to an illumination device, an image reading apparatus having the illumination device and an information processing system having the image reading apparatus, and more particularly to an information processing system such as copying apparatus, facsimile apparatus, scanner and electronic blackboard, an image reading apparatus used therein and an illumination device suitably used in the reading apparatus.
2. Related Background Art
In the prior art a reading apparatus of an information processing system such as a facsimile machine or an electronic copying machine, a discharge lamp such as a fluorescent lamp or an LED array having a number of LED chips an array has been used as an illumination device. Recently, as the facsimile machine is used even in a home, a compact and low cost product is required and many products use the LED arrays.
In order to improve an output image quality, a function capable of outputting a multitone image which images a color document read information without drop while using monotone image has been replacing a conventional binary image system, and an improvement of color discrimination ability for color image read information in an image reading apparatus is required to cope with the future colorization.
A light source of an image reading apparatus to improve the color discrimination ability of the color document uses a plurality of LED's having a predetermined wavelength range and it is desired to emit a linear light beam from each of the LED light sources. An example of the image reading apparatus having the illumination device which uses such LED light sources is shown in FIGS. 1A to 1C.
FIG. 1A shows a sectional view of a photoelectric conversion element array of the image reading apparatus as viewed along a main scan direction, and FIG. 1B shows a side elevational view of the image reading apparatus as viewed along an arrow A shown in FIG. 1A.
In FIGS. 1A to 1C, numeral 10 denotes a light transmissive sensor substrate on which a plurality of photoelectric conversion elements formed by using a thin film semiconductor layer such as amorphous silicon or polycrystalline silicon are arranged in one dimension. A protective layer, not shown, is provided on the light transmissive sensor substrate 10 to protect the photoelectric conversion elements, not shown, from damage due to relative movement of an original.
The light transmissive sensor substrate 10 is packaged on a light transmissive packaging substrate 15 by bonding and electrically connected with a drive circuit, not shown, which is also packaged, by wire bonding.
Numeral 30 denotes an illumination means (light source) which comprises LED light sources 41R and 41G. Numeral 3 denotes a light transmissive member such as a silica rod having a circular cross-section, numeral 4 denotes an incident plane through which a light beam emitted from the illumination means 30 is applied to the light transmissive member 3, and numeral 5 denotes a scatter and reflection area for scattering and reflecting the light beam propagated through the light transmissive member 3 to take out of the light transmissive member 3. The scatter and reflection area 5 is formed by roughening a portion of the surface of the light transmissive member 3 or applying light diffusion reflective paint. Numeral 6 denotes a reflection plane formed at an end plane opposite to the illumination means 30 of the light transmissive member 3. The reflection plane may be formed by vapor depositing a metal such as aluminum on the surface of the end of the light transmissive member 3, or applying a light diffusion reflective paint, or it may be a separate member. A sectional shape of the light transmissive member 3 is commonly square or rectangular.
A read position of an original 100, an arrangement position of an illumination window of the light transmissive sensor substrate 10 and an optical axis along the array of the scatter and reflection area 5 of the light transmissive member 3 are set such that they are in a vertical plane passing through a read position of the document sheet 100.
A light beam L emitted from the LED light sources 41G and 41R of the respective light emission wavelengths of the illumination means 30 and directed into the light transmissive member 3 from the incident plane 4 of the light transmissive member 3 repeats the reflection at an inner surface of the light transmissive member 3 and propagates therein, and reaches the opposite plane to the incident plane 4, where it is again reflected and propagates in the light transmissive member 3. While it repeats the reflection, it reaches the scatter and reflection area 5 where it is diffused and a portion I.sub.1 thereof is emitted from an exit plane located to oppose the area 5 and it passes through the light transmissive packaging substrate 15 and the illumination window in the light transmissive sensor substrate 10 and irradiates the document sheet 100. Another portion I.sub.2 of the diffused light beam is directed to the exit plane obliquely so that it is totally reflected and propagates in the light transmissive member. It repeats the propagation and finally reaches the incident plane 4 where it is emitted.
The light beam which irradiates the document sheet 100 is reflected by the original 100 and directed to the photoelectric conversion elements on the light transmissive sensor substrate 10 where it is photoelectrically converted to produce an image read signal which is outputted.
FIG. 1C shows distributions G and R of document plane illumination in a main scan direction of the photo-electric conversion element array of the light sources 41G and 41R having the respective light emitting wavelength ranges when the image reading apparatus shown in FIGS. 1A and 1B is used.
In the foregoing example, as described above, the LED light sources 41G and 41R having a plurality of light emission wavelength ranges are used as the illumination means 30.
There are various sorts of LED light sources and they cannot be generally discussed, but an LED chip of a surface packaging type which attains further compaction and is convenient for packaging has been developed recently. FIG. 2 shows a surface packaging type LED light source. In FIG. 2, numeral 81 denotes an LED chip, numeral 82 denotes a substrate, numeral 83 denotes a reflection frame, numeral 84 denotes a light transmissive resin, and numerals 85 and 86 denote electrodes formed on a surface of the substrate 82. The size of the LED light source is less than 2 to 3 mm in length and less than 2 mm in height. Since the electrodes 85 and 86 are taken out via a side of the substrate 82, it can be packaged by merely placing creamy solder on the printed packaging substrate and heating (reflowing) it by a reflow oven. Thus, efficient packaging is attained. Accordingly, it is desirable to use such LED light source as a linear light source.
Since such an LED light source has a light emission directivity as shown in FIG. 2, when a document sheet is to be illuminated by conducting, reflecting and diffusing the light by the light transmissive member 3 as shown in FIGS. 1A and 1B, the illumination distribution is not uniform, that is, the illumination is high in the area closer to the light source 30 and low in the other area. Thus, it poses a problem in the uniformity of the illumination distribution.
This is due to the fact that the light beam obliquely emitted from the LED light source 30 is directly applied to the area 5 of the light transmissive member 3, scattered there and taken out of the light transmissive member 3.
Another problem arises in that the illumination to the document sheet of different light emission wavelength ranges has different factors depending on the longitudinal position of the document sheet. This is due to the difference in the ratios of incident light beams, that is, the distributions, along the longitudinal side of the scatter and reflection area depending on the light emission wavelength range. (In FIG. 1C, solid line and broken line represent relative illumination distributions of a green ray and a red ray, respectively.)
Where special means to compensate such irregularity of illumination is to be provided, the mechanism is complicated and its cost increases.
FIG. 3 shows a perspective view of another example of the linear light source. The light sources 30 are arranged at the opposite ends of an elongated transparent member which is a light conductor 3.
Namely, in FIG. 3, numeral 3 denotes an elongated transparent member (light conductor), and numeral 11 denotes a direction of light emission. A cross section of the elongated transparent member 3 is constant, and it is mirror finished on a plane other than a light emitting plane. A light is emitted from an LED chip 71 on a substrate 45 and it is directed through an end of the elongated transparent member 3. The light is reflected directly or by the mirror finished reflection plane so that it is emitted from the elongated transparent member 3. In FIG. 3, a plurality of LED chips having different light emission wavelengths are shown by an LED chip 71.
FIG. 4 shows a front view as viewed in the direction D of FIG. 3 and a light intensity distribution on an illumination plane (not shown). As shown, a uniform light intensity is attained between a and c but a total light intensity is low and a difference from the light intensity near the light source is large. Numerals 10a, 10b and 10c show sections at points a, b and c of the elongated transparent member 3, numerals 44a, 44b and 44c denote light intensity distributions, and a hatched area (the plane excluding the light emission plane and the light incident plane of the elongated transparent member 3) denotes a mirror finished plane.
In FIG. 4, the light intensity distribution is shown for one of the plurality of LED chips as being representative.